The present invention relates to a process for leaching of precious metals (e.g., gold and/or silver) from particle-shaped, solid materials, particularly ores and ore concentrates. The process includes the use of a cyanide-containing alkaline leach solution in the presence of a peroxo compound releasing oxygen. The present invention also relates to a process for leaching of such precious metals from ores, ore concentrates and waste material from previous incomplete leachings by grinding of the material to be leached in the presence of a cyanide-containing leaching solution which has a pH value of 8 to 13 and which contains a peroxo compound.
Leaching of precious metals involves the formation of cyano complexes (particularly complexes with gold and/or silver) from ores, ore concentrates, and other particle-shaped, solid materials. Such solid material may be available, for example, from mining waste material (or tailings), from previously incomplete leaching operations, or from electronic scrap. A leaching process using a cyanide-containing alkaline leach solution and an oxidizing agent, normally atmospheric oxygen, has long been known. Although air is widely used as an oxidizing agent for so-called leaching by agitation, as well as for heap leaching, there have been many attempts to increase leaching speed and the yield of precious metals (i.e., the degree of extraction) by using agents for releasing oxygen or other oxidizing agents.
Hydrogen peroxide has proved to be a suitable agent for increasing the oxygen concentration of the leach solution, thus accelerating leaching and increasing the degree of extraction. Oxygen in dissolved form is released by the decomposition of the hydrogen peroxide during leaching (see for example U.S. Pat. Nos. 732,605 and 3,826,723, and Canadian Patent 1,221,842; see also Japanese Kokai 01-270512). These processes were not considered viable solutions for a long time due to the large amount of hydrogen peroxide and sodium cyanide being used.
The problem that prevented a practical utilization of these processes (i.e., the excessive consumption of cyanide and hydrogen peroxide) was solved by the process mentioned in German Patent DE-PS 36 37 082. In this patent, a process is described wherein the addition of the aqueous H.sub.2 O.sub.2 solution is adjusted via the concentration of the oxygen dissolved in the cyanidic leach solution, whereby the O.sub.2 concentration should be situated in the range of 2 to 20 mg per liter (preferably 7 to 13 mg per liter).
In order to solve the same problem, German Patent DE-PS 38 01 741 (U.S. Patent No. 4,971,625 which is incorporated by reference in its entirety) suggests a different approach. In this patent, special molar ratios of hydrogen peroxide to cyanide are maintained, and specific pH-ranges are maintained, along with the addition of the total amount of hydrogen peroxide at the beginning of the leaching.
When using the process described in German Patent DE-PS 36 37 082 in gold mines, an unexpected high consumption of chemicals and/or an insufficient increase in the gold yield and/or a reduction of the leaching period occurred in some cases. This happened in spite of careful adjustment of the hydrogen peroxide dosage. An improvement could be obtained in some instances by simultaneous use of dissolved or solid decomposition catalysts for hydrogen peroxide (as described in European Patent Application EP-A 0 358 004). The concentration of free hydrogen peroxide could thereby be kept sufficiently low, whereby the consumption of cyanide and hydrogen peroxide could be lowered.
The chemical consumption, the maximum degree of extraction, and the pertinent leaching period apparently depend upon the chemical and physical properties of the material to be leached in some manner that is not immediately predictable.
Although the prior art processes which use hydrogen peroxide as an oxygen releasing source are often superior to the process using conventional air or oxygen-gasing techniques, a great interest persists in trying to further decrease chemical consumption and, during the shortest possible time, to obtain a maximum degree of extraction.
In British Patent Application GB-A 2,219,474, a further process of leaching gold-containing materials by using a diluted aqueous alkaline cyanide solution in the presence of an in situ compound releasing oxygen (e.g., a peroxide of a divalent metal, especially calcium peroxide) is described. The peroxide compound is applied either as a solid or a slurry produced from aqueous hydrogen peroxide and a divalent metal oxide or hydroxide. It is added to the ore pulp for leaching by agitation and it is added to the leaching material during filling up of the heap for heap leaching.
During reproduction of the process in British Patent Application GB-A 2,219,474, it appeared that, due to the relatively high chemical consumption, the plausibility of this process is rather restricted. More particularly, the consumption of the peroxo compound (in this instance the moles of calcium peroxide per ton ore for leaching by agitation) was significantly higher than the amount required for leaching according to the process of German Patent DE-PS 36 37 082 which utilizes hydrogen peroxide.
For some time now, certain mines have been using the so-called cyanidation-in-mill technique. In this method of leaching, the cyanide is added before or during grinding and the residence time of the ore in the mill is used to shorten the leaching time in the tanks used for agitation leaching. Whereas passable results can actually be obtained in the mill in the leaching of fully oxidized ores, this method is not really suitable for the leaching of sulfidic ores because, in their case, reductive conditions prevail in the mill and too little, if any, oxygen is available for leaching. To make sulfide-containing ores more accessible to the cyanidation-in-mill technique, oxidizing agents have also been used by various mines. Whereas no improvement could be obtained with pure oxygen, the gold yields in the mill could be increased and the consumption of NaCN reduced by using hydrogen peroxide (Chemical Abstracts 102(14): 117216r; Smith, M. E., et al. in Proc. SME Fall Meeting 1983, 41, 43-49, ed. Hiskey, J. Brent, Soc. Min. Eng. AIME: Littleton, Colo.).